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Trade-offs for Cross-sectional Reallocation on Urban and Suburban Roads


Transportation professionals strive to provide safe and reliable service for automobiles, freight, pedestrians, bicyclists, and transit in balance with the desire to enhance communities and minimize impacts to the environment. Designs for urban and suburban streets must effectively serve all transportation modes and provide an appropriate operational and safety balance among those modes.

The allocation of space within the travelled way (i.e. the road’s cross-section) provides the elements that allow for the movement of vehicles, transit, bicycles and freight. Roadway design guidance exists for the individual elements of the traveled way, such as lane width, bicycle lanes, on-street parking, medians, midblock crossings and pedestrian refuge islands, and transit stops. However, current design guidance may not reflect the complex trade-offs transportation professionals must consider for roadways in varying contexts. Also, in constrained locations certain combinations of minimal widths for adjacent elements may have undesired consequences. For example, combining minimum width travel lanes adjacent to minimum width bicycle lanes and parking.

Many transportation agencies have reallocated existing cross-sectional pavement space through resurfacing projects as an effective and low-cost strategy to improve safety and develop multi-modal corridors. There are many options for reconfiguring a roadway to add bicycle lanes, sidewalks, on-street parking or transit stop pull-outs. Oftentimes these types of projects are referred to as “Road Diets”. There are many potential benefits to reallocating the use of pavement space. Providing for a two-way left turn lane (TWLTL) can greatly reduce the risk of rear-end and angle collisions for mid-block left-turning motorists. Decreasing the number of road lanes reduces pedestrian and bicycle exposure to traffic when crossing the street and the extra space can be used to add pedestrian refuge islands, widen sidewalks, and enhance local businesses along the corridor by providing outside areas for showcasing merchandise or providing seating for restaurant diners. Cross-sectional reallocations can also provide opportunities to add bicycle lanes to the street or for installing bus pullouts so transit users can enjoy safer stops that do not hinder the flow of traffic. Cross-sectional reallocations can be relatively inexpensive to implement, especially when done through a resurfacing project.

Although cross-sectional reallocations can offer significant multi-modal benefits, they may not be feasible or appropriate in all locations. Transportation agencies must consider numerous factors in terms of feasibility and the overall objectives of the corridor for balancing service among modes. Since there are likely to be positive and negative effects associated with road configuration choices, it is critical to consider the potential outcomes for all users and assess both the beneficial and detrimental outcomes in relation to overall goals and objectives of the project. However, there is limited guidance available to transportation professionals on the key considerations and how to evaluate trade-offs when screening and evaluating cross-sectional reallocations on projects.


The objective of this research is to develop enhanced guidance on the key considerations when evaluating and designing cross-sectional allocations on urban and suburban streets. This guidance should be based on a combination of contextual considerations and quantitative operational and safety-performance information and should examine potential mitigation strategies to minimize negative effects. The enhanced guidance should consider the volume of traffic individually within each mode for the combination of modes being considered in both the before and after conditions. An evaluation framework, including appropriate performance measures is desired.

The research should examine the safety and operational aspects of a sampling of representative cross-sectional reallocations over a range of design parameters (i.e. lane widths, speeds, and varying cross-sectional elements, and various volume combinations for the modes being considered). The guidance should also consider driver expectations and behaviors over a range of traffic conditions. Development of this guidance will require the examination of field data and site observations supplemented with safety modeling considerations and traffic operational simulations.

The research should examine both performance and feasibility aspects related to cross-sectional reallocation. The issues associated with evaluating these proposals are complex and need research to offer modern perspectives and insights on safety performance and economic trade-offs associated with such roadway changes. The final report should include enhanced guidance targeted to design practitioners.

Related Research:

Ongoing – NCHRP 03-112: Operational and Safety Considerations in Making Lane Width Decisions on Urban and Suburban Arterials

Safety and Operational Analysis of 4-Lane to 3-Lane Conversions (Road Diets) in Michigan, Lyles, Richard W., M. Abrar Siddiqui, William C. Taylor, Bilal Z. Malik, Gregory Siviy, and Tyler Haan, Lansing, Michigan: Department of Civil and Environmental Engineering, Michigan State University (2012)

NCHRP Synthesis 422: Trade-Off Considerations in Highway Geometric Design, A Synthesis of Highway Practice (2011)

Potts, I. B., D. W. Harwood, and K. R. Richard, Relationship of Lane Width to Safety on Urban and Suburban Arterials, Transportation Research Record: Journal of the Transportation Research Board, No. 2023, Transportation Research Board of the National Academies, Washington, D.C., 2007, pp. 63–82.

Road Diet Informational Guide, FHWA (2014)


Support for transportation agencies to implement the research results could be in the form of webinars and materials for an instructor led-training course. Other activities to help facilitate dissemination and implementation of the guidance would include presentations at national conferences and practitioner peer exchanges aimed at specific target audiences.


This research need has been identified as a high priority by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their June 2018 combined meeting. The research is needed to provide enhanced guidance that will directly affect future design practices for use nationally.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
Research Priority:High
RNS Developer:Jeremy Fletcher (Florida DOT), Larry Sutherland (WSP), Susan Keen (Virginia DOT), Mark Doctor (FHWA), Pete Jenior (Kittelson & Assoc)
Source Info:This problem statement was developed in connection with the June 2018 mid-year joint meeting of the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design (AFB10), and the TRB Committee on Operational Effects of Geometrics (AHB65).
Date Posted:09/21/2018
Date Modified:12/31/2018
Index Terms:Alternatives analysis, Highway design, Multimodal transportation, Vehicle mix, Rural highways, Urban highways, Highway facilities for motorized users, Highway facilities for nonmotorized users,
Cosponsoring Committees: 
Pedestrians and Bicyclists
Operations and Traffic Management
Planning and Forecasting

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